Bt corn pollen can hurt monarchs.

Pollen wafts from a field of corn genetically engineered to make its own insecticide in amounts sufficient to kill monarch butterfly caterpillars nearby, according to an Iowa study.

The corn variety examined carries a gene from Bacillus thuringiensis for an especially strong punch of the bacterial toxin. The corn sheds pollen at worrisome concentrations up to 10 meters from the field, report Laura C. Hansen Jesse and John J. Obrycki at Iowa State University in Ames. They presented an early analysis of these data last year (SN: 12/18&25/99, p. 391), and OECOLOGIA released the full paper online in August.

For the past year and a half, debate has raged over whether Bt corn's built-in pesticide hurts harmless insects. A Cornell University team ignited the debate by dusting leaves with Bt corn pollen and feeding them to monarch caterpillars in the lab (SN: 5/22/99, p. 324). About half the caterpillars died. Critics complained that the pollen dustings didn't reflect natural levels.

Jesse and Obrycki let nature do the dusting, however. They then fed the leaves to caterpillars in a laboratory.

Chicago researchers testing another caterpillar found it unharmed by a corn variety with low Bt production (SN: 6/10/00, p. 372). Other researchers have indications that wild monarch caterpillars may be able to avoid high concentrations of Bt.